The present invention relates generally to the field of digital radio communications. More specifically, the present invention relates to a direct conversion transmitter architecture utilizing a quadrature generator system to produce low noise transmission signal.
As a new generation of radiotelephones is introduced, it is expected that the new phone is more capable and more efficient. For example, the new phone may be able to support multiple systems and standards, which were traditionally incompatible, such as time-division-multiple-access (TDMA), code-division-multipleaccess (CDMA), and global-system for mobile (GSM), and be able to provide more talk time without increasing in size. These new radiotelephones may utilize a direct conversion architecture instead of a traditional heterodyne architecture. A direct conversion transmitter employs a local oscillator (LO) frequency that is substantially equal to the radio frequency (RF) transmission signal thereby eliminating an intermediate frequency (IF) circuit.
A direct conversion transmitter system to produce a transmission signal is generally comprised of an LO, a phase locked loop (PLL), a quadrature generator, a modulator, a power amplifier (PA), and one or more filters. The LO, coupled to the PLL, produces a signal with a frequency that is substantially equal to the frequency of a desired RF transmission signal. The quadrature generator is coupled to the LO and the modulator. It receives the signal from the LO, and produces two signal that have a phase offset of 90xc2x0 relative to each other and have the same frequency as the LO. Each of the signals is fed to a mixer in the quadrature generator. Each mixer also receives a signal from the modulator, which contains the information to be transmitted. The resulting signals from the mixers now contain the information to be transmitted, and have center frequencies equal to the desired transmission frequency, but at a phase offset of 90xc2x0 relative to each other. These signals are combined at the output of the quadrature generator to produce a transmission signal. The PA is coupled to the quadrature generator, and receives the transmission signal and amplifies it. The amplified signal may go through a filter to reduce noise or spurious outputs outside of the transmission band. A typical radiotelephone may employ a bandpass filter following the PA to reduce undesired noise present at the antenna in different portion of RF spectrum to meet various standards"" regulations and specifications. This post-PA filter significantly reduces the undesired noise, however, it also attenuates the desired transmission signal, and it is normally large relative to the overall size of a modern radiotelephone. If the radiotelephone were designed to support multiple standards, such as TDMA, CDMA, and GSM or any combination of them that may require different sets of frequencies, the phone would be required to have multiple post-PA filters to meet each of the standards. Having multiple post-PA filters would make the phone larger and more expensive.
The post-PA filter may be eliminated by producing a RF signal at the desired frequency and power that meets the noise requirement imposed by the standard at the output of the PA. By eliminating the filter, the loss through the filter, which the PA must overcome, would be eliminated thereby increasing the overall efficiency of the transmitter. For example, if the loss after the PA were 3 dB and the desired power output were 30 dBm (or 1 Watt), then in order to overcome the loss, the PA would be required to produce 3 dB more power, or 33 dBm (or 2 Watts), at the output of the PA. One half of the power produced would have to be lost in order to reduce the undesired noise. The loss would vary depending on the requirement by the standard and the type of a filter used, however, a significant portion of the RF power produced by the PA would still be lost. By eliminating the filter after the PA, the output power requirement of the PA would be reduced. By reducing the output power requirement, the power consumption by the PA would be reduced, which would lead to a longer talk time, and a smaller and less expensive PA device might be used. By eliminating the filter, or multiple filters, and utilizing a smaller PA device, the overall size of the phone may be reduced.
Prior arts concerning the direct conversion architecture have mainly concentrated on the apparatus and method for generating quadrature signals that might be used for multiple bands and for multiple systems. However, these prior arts have not considered the noise optimization aspect of a direct conversion system for a transmitter to the level at which the post-PA filter would be unnecessary for a RF transmission signal.
The present invention describes an apparatus capable of producing a RF transmission signal for a radiotelephone low enough in noise without requiring a post-PA filter.